1. Field of the Invention
The present invention relates to a method for welding a shadow mask, and more particularly, to a method for welding a shadow mask in a flat CRT(Cathode Ray Tube).
2. Description of the Related Art
Recently, CRTs used as TV receivers or image displays have been provided with a flat surface which can eliminate image distortion, minimize reflection of external light, and maximize a visual area. As shown in FIG. 1, a conventional flat CRT is provided with a panel 1 having an inside surface coated with a fluorescent material 1a, an inner shield (not shown) fixed to the inside surface of the panel 1 to provide shielding from external magnetic fields, a funnel 5 sealed to an inside surface periphery of the panel 1 by frit glass, and a neck portion 5a positioned at a rear of the funnel. An electron gun 6 is provided in the neck portion 6 for emitting three colored electron beams of R, G, B. A deflection yoke (or deflection coil) 7 is provided on an outer circumference of the neck portion 5a for deflecting the electron beams in horizontal or vertical direction. The panel 1 is provided with safety glass 2 fixed to a front surface thereof by resin to prevent bursting of the cathode ray tube, and a rail 3 is fixed to an inside surface of the panel 1. A shadow mask 4 is attached to the rail 3 for selecting a color from the electron beam. The rail 3, a rectangular frame, is used to attach the shadow mask 4 to the inside surface of the panel 1 with an appropriate distance between the shadow mask 4 and the panel 1. The rail 3 has a side facing the panel 1 with a groove 3a formed therein containing frit glass, and a contact surface 3b on the other side thereof contacting the shadow mask 4. When molten frit glass fills the groove 3a, and the rail 3 is attached to the inside surface of the panel 1, the rail 3 is fixed to the inside surface of the panel 1 when the molten frit glass solidifies. The shadow mask 4 is then placed on the contact surface 3b while the shadow mask 4 is subjected to a tensile stress and is bonded to the rail 3 by welding. Since the shadow mask 4 for use in a conventional flat CRT is formed of a thin plate (around 0.1 mm thick), the use of resistance welding is not desirable because the resistance welding causes a large mechanical deformation in the shadow mask and forms a large heat-affected zone. Therefore, laser beam welding is used to weld the shadow mask in a flat CRT, which provides fast and high quality welding, with a relatively small deformation and heat-affected zone of the object being welded.
However, despite of the use of laser welding, the shadow mask 4 may still be damaged and deformed during the welding, as follows.
In laser beam line welding, the thin plate may be pushed forward in the direction that the welding is progressing, causing the plate to be welded at a position deviated from a desired position. In order to avoid such a defect when a spot welding is used, there may be plastic deformation caused by thermal stress in the vicinity of welding spot if the welding is carried out without accurately calculating a size of the welding spot and a distance between the welding spot. The plastic deformation causes of distortion because of a difference of thermal expansion coefficients when heating occurring during fabrication of the flat CRT dissipates.
Even though laser welding is employed, welding the shadow mask 4 and the rail 3 by the aforementioned one run welding causes many defects, and results in low productivity since a high level of skill is required to prevent the defects.